The present invention relates to a hot rolling method and an apparatus suitable for carrying out the method of the invention. More particularly, the invention is concerned with a hot rolling method in which an intermediate heating step is employed in the rolling line so as to heat the portion of the rolled material which has been cooled down below Ar.sub.3 transformation temperature as the rolling proceeds, thereby attaining higher uniformity of the rolled product, as well as an apparatus suitable for carrying out this method.
Generally, hot rolling, particularly hot rolling of a hot strip, comprises heating in a heating furnace a material to be rolled, and rolling the material by use of a plurality of rough hot rolling stands and a plurality of stands for finishing tandem hot rolling adapted to roll the material into a predetermined size.
The material under hot rolling, particularly the rough-rolled material (referred to as "bar", hereinunder) having a large heat radiation area, exhibits a remarkable temperature decrease at the edges thereof, due to a stagnation thereof in the line of hot-rolling or due to a descalling by use of pressurized water, resulting in defects such as duplex grain structure or abnormal profile in the hot strip after the final hot rolling.
FIG. 1 shows a partial schematic sectional view of such a hot strip taken along the breadth of the strip, illustrating the structure of the strip. In this Figure, a duplex grain region is denoted by a numeral 1, while a numeral 2 denotes a fine grain region. Symbols (a) and (b) represent, respectively, the thicknesses of the duplex grain region at the upper and lower sides of the strip, while (t) shows the thickness of the strip.
The duplex grain region has to be severed because it impairs the quality of products. The presence of such duplex grain region, therefore, impractically reduces the yield of the product.
In order to obviate this problem, various countermeasures or methods have been proposed and adopted as follows:
(1) An ordinary countermeasure in which the material is over-heated in the heating furnace so as to effect overcompensation for possible temperature drop.
(2) Local compensation heating of the edges of the bar or skid marks portions occurring in the heating furnace, during the rough hot rolling or after the rough hot rolling but before the finish hot rolling.
(3) Local compensation heating of the edge portion in the course of finish hot-rolling as proposed in Japanese Patent Unexamined Publication No. 160502/1982.
The ordinary method (1) mentioned above is not preferred because it requires over-heating of the whole of the material and, hence, causes a large loss of energy. It is known that in the method (2) there occurs a smaller loss of energy as compared with the method (1) and the method (3) permits a further reduction in the energy loss. In the methods (2) and (3), however, the edges or skid mark portions of the material are heated in the intermediate stage of the hot-rolling substantially to the same temperature as the center portion of the material, so that the finish hot rolling is completed while whole portion of the material is still at temperatures not lower than the Ar.sub.3 transformation temperature.
With this knowledge, the present inventors have conducted a test under the conditions described in Table 1, using a hot rolling line having seven finish hot rolling stands F1 to F7. In this test, the edges of the material, which had been cooled down below the Ar.sub.3 transformation temperature in the course of the finish hot rolling, were heated by electric induction heating to a temperature above the Ar.sub.3 transformation temperature and equal to the temperature of the breadthwise central portion of the material. The material was then subjected to a further finish hot rolling which was completed while the whole portion of the material still exhibits temperature above the Ar.sub.3 transformation temperature. A microscopic observation of samples taken from the finished material showed presence of duplex grain structure in the edge portions. Thus, this method proved to be still unsatisfactory as a method for preventing the duplex grain structure from occurring.
TABLE 1 ______________________________________ Number of finishing stands 7 Heating position Between stands F1 and F2 Edge temperature 745.degree. C. (minimal surface temp. before heating) Edge temperature 846.degree. C. (minimal temp. after heating Temp at breadthwise center 878.degree. C. Contents C: 0.04%, Mn: 0.21% Ar.sub.3 transformation temp. 824.degree. C. Final thickness 2.5 mm Finish hot rolling 827.degree. C. completion temp. ______________________________________